Since the discovery of carbon nanotubes in 1991 by Iijima, Nature, 354, pp. 56-58 (1991), extensive research has been carried out involving the synthesis, chemistry, and manipulation of these materials. Carbon nanotubes are electrically conductive along their length, chemically stable, and capable, individually, of having a very small diameter (much less than 100 nanometers) and large aspect ratios (length/diameter).
Force sensors made of resistive inks or materials that can measure applied force by changes in resistance have been known in the art since around 1968 as evidenced by U.S. Pat. No. 3,806,471, issued to Mitchell. Mitchell used a volumetric dispersion of at least partially conductive particles such as molybdenum disulfide in a compressible insulating elastomeric material such as RTV silicon rubber. When a force is applied to compress the material the particles touch and increase the number of conductive paths through the bulk material thereby decreasing the resistance of the material. When the force is removed the material conforms to its original shape and returns to its original resistance. The change in resistance is a bulk material effect and not a surface effect, and as such does not work well in thin film applications. Additionally, the high loadings of conductive particles in the elastomeric material will reduce the elastomeric properties of the bulk material.
U.S. Pat. Nos. 4,489,302 and 4,314,227, both issued to Eventoff, disclose an improved force sensor based on the pressure dependent contact area between a conducting surface and a semiconducting surface having a multiplicity of microprotrusions of the semiconducting material. In one embodiment, the loading of the semiconducting molybdenum disulfide material in the resin surface is between 46% and 63%. Such high loadings of filler will reduce the physical properties of the resin binder, such as flexibility. Eventoff teaches in some of embodiments that the flexible film used as the substrate is MYLAR. The substrate films in Eventoff can conform to a two dimensional surface but cannot conform to a three dimensional surface well without creasing.
Maness et al., in U.S. Pat. No. 4,734,034, disclose a flexible force sensing pad suitable for measuring dental occlusions. Maness et al. teach a flexible high spatial resolution force sensor pad can be fabricated using a matrix of force sensor cells. The force sensor cells are composed of a plurality of flexible parallel electrodes supported by a thin flexible film substrate to provide a set of row electrodes; a second plurality of flexible parallel electrodes supported by a second thin flexible film substrate to provide a set of column electrodes. At least one of the two sets of electrodes is coated with a thin film resistive material, preferably an insulating vinyl ester binder with titanium dioxide filler and butyl cellosolve acetate solvent with a conductive ink containing graphite, vinyl resin, and butyl cellosolve acetate; and oriented such that the two electrode sets face each other to form a matrix of rows and columns that defines a measuring space suitable for measuring dental occlusions. The two sets of electrodes are separated by a resistive layer such as talcum powder to prevent contact until there is an applied force. The response of the force sensor cells as taught by Maness et al. is of a binary nature which either senses an applied force or no applied force but cannot discriminate between small changes in applied force. Maness et al. teach in the preferred embodiment of the invention, the thin film used as the flexible substrate is polyester film such as MYLAR, or similar films such as KAPTON, manufactured by DuPont. The substrate films of Maness et al. can conform to a two dimensional surface but cannot conform to a three dimensional surface well without creasing.
U.S. Pat. No. 4,856,993 also issued to Maness et al., is said to provide an improvement on U.S. Pat. No. 4,734,034, wherein part of the resistive coating between the conductive plurality of flexible parallel electrodes in rows and columns is composed of a graphite-molybdenum disulfide based ink in an acrylic binder. The separating resistive layer (talcum) between the two sets of electrodes is also eliminated. Maness et al. teach the thin film used as the flexible substrate is polyester film such as MYLAR, or similar films such as KAPTON. The substrate films of Maness et al. can conform to a two dimensional surface but cannot conform to a three dimensional surface well without creasing. The pressure sensing resistive layer is composed of high levels of graphite and molybdenum disulfide that reduces the flexibility and mechanical properties of the layer.
U.S. Pat. No. 5,296,837 issued to Yaniger, discloses a pressure transducer based on at least one three-dimensional stannous oxide textured resistive layer juxtaposed to at least one of two conductive layers containing a circuit to measure the resistance between the two conductive layers. The measured resistance is a function of the resistance layers contact area or pressure perpendicular to the resistive layers. In one embodiment, the pressure transducer resistance is dependent on the area of contact between the two juxtaposed resistive layers. At low pressure, just the peaks of the stannous oxide particles of the textured surfaces are in contact giving a high resistance. As the pressure increases, the two resistive layers bend to conform to the valleys between the stannous oxide particles and increase the contact area between the resistive layers. One advantage to this form of pressure transducer is it relies on a surface effect and not a bulk effect and thus may be well suited to thin film applications. However, in the preferred embodiment of Yaniger, the levels of stannous oxide filler can range from 16% to 80% of the conductive resin layer which would have a considerable effect on the physical properties of a thin film, such as flexibility. Yaniger teaches the thin film suitable as the flexible substrate is a polyester film such as MYLAR. Similarly to other referenced patents herein, the substrate films of Yaniger can conform to a two dimensional surface but cannot conform to a three dimensional surface well without creasing.
U.S. Pat. Nos. 6,964,205 and 7,258,026, both issued to Papakostas et al., disclose a method for fabricating a plurality of sensor cells to measure a parameter applied to a surface. Papakostas et al. try to address the shortcomings in the above-referenced patents on flexibility in the film substrates and flexibility in the resistive measuring layer. Papakostas et al. teach that flexibility and mechanical isolation between cells can be achieved by using spiral-like conductive traces which wrap around at least a portion of the sensor element to interconnect the sensor elements. Papakostas et al. further teach slits or cut-outs in the substrate between the sensor elements will allow sensor elements to move independently of one another. Cut slits in the film will improve the overall flexibility of the film and independent movement of the individual sensor elements, but the sections of the film and sensor cells will still be subject to stress cracking in areas of high flexing loads.
It is known to those skilled in the art that non elastomeric films or substrates in applications that involve extensive flexing will undergo stress cracking over time. The use of sensor pads or films in applications such as shoes, wheelchairs, hospital beds, and car scats has thus been limited due to the robustness of the materials of construction.
Therefore, a need continues to exist in the art for improved materials for sensing one or more parameters such as pressure, temperature and moisture. A need also exists for a highly flexible force sensing cell and sensor array on a highly flexible and elastomeric film substrate. The sensing material should preferably be flexible to permit its use in a wide variety of applications.